1. Field of the Invention
The present invention pertains to systems and methods for characterizing mechanical properties when subject to multiple strains and strain rates.
2. Discussion of Related Art
With the rising cost of fuel, automobile manufacturers are looking for ways to improve fuel mileage. One way to do this is to use lightweight materials, such as alloys of Magnesium (Mg), when manufacturing an automobile. In the case of using Magnesium alloys, automobile manufacturers desire to use them because of their high strength-to-weight ratios. The application of magnesium alloys in the automobile industries will satisfy the goal of vehicle weight reduction and fuel efficiency improvement.
Before a material is used in an automobile, a number of information is needed. For example, mechanical properties under impact, damage and failure characterization, material and failure models and Finite Element Models (FEM) technology need to be determined. In addition, methods for material characterization under impact need to be developed. In the case of Magnesium alloys, little information regarding crashworthiness of the material is available. With that said, there are differences between the mechanical properties of conventional materials, such as High-Strength Low-Alloy Steel (HSLA), and Magnesium alloys. For example, as shown in FIGS. 1(a)-(f) the impact energy dissipation in HSLA during a crashworthiness test occurs by a different process than in a Magnesium alloy tube as shown in FIGS. 2(a)-(f). Faced with the difference in mechanical properties, current automobile designs that employ lightweight materials, such as Magnesium alloys, result in overdesigned components in order to compensate for the uncertainties in the deformation and failure mechanisms. Such overdesigning can be avoided by understanding the initiation and evolution of internal state and failure processes in the lightweight materials as functions of loading type and loading rates. Reducing uncertainties in component design would greatly improve the overall vehicle system reliability and enable weight reduction of the automobile.
Another property to test for a material is their response at different strain rates. Testing at low (quasi static) rates is performed in order to observe the situation when the system is in equilibrium at all times. At the other end of the spectrum, fast rate tests have been performed wherein a single impact pulse travels through the system. A more problematic area of interest is so-called intermediate rate tests performed in the range of between 1/s and 1000/s with multiple wave reflections in the system. This area is important because maximum strain rates are in the interval of 10-1000/s for automotive crashes. For intermediate rates, it is difficult to establish dynamic equilibrium in the sample and the system and it is a fact that intermediate strain rate tests have not been established.
With the above said regarding strain rates measurements, it is important to apply certain principles to the measurement of strain rates for lightweight materials. Such principles include: reducing mass in the system; developing lightweight load cells and sensors; understand and control oscillations in the system; and combine multiple measurement techniques for the same data.
One way to understand the structure of materials, such as lightweight materials, is to study the response and microstructure changes in the design of the materials. Material mechanical response and microstructure changes, such as microstructure defect evolution, are often dependent on the levels of imparted strains and strain rates. The changes in microstructure can provide understanding about processes of material mechanical degradation that can lead to structural failure. Many materials exhibit different mechanical response when loaded by different deformation rates. This property is called strain rate sensitivity and is conventionally examined by using multiple specimens and tensile tests.
For tensile test configurations, standard dog-bone specimens (ASTM E8) are used. For the sample geometry of standard dog-bone specimens, uniform deformation is achieved within the gage section and the measured strains and strain rates are related to the displacements in this region. In order to characterize the evolution of an internal state of a material at different rates of strain, it is required to instantly stop (interrupt) the deformation from a current loading speed. This interruption of the deformation is possible at very low loading speeds, but at velocities necessary to generate strain rates of 1/s and higher, the inertia of the loading equipment and control system makes this task impractical using conventional test methods. Performing strain-interrupted tests using dog-bone specimens becomes exceedingly difficult, if not impossible, at high rate testing speeds. Complicated testing fixtures have been proposed and have shown to be impractical at high rates of strain. These fixtures add extra mass in the loading train of the testing equipment and consequently introduce additional oscillations that reduce the quality of the measurements. Low (quasi-static) rate tests—entire system is in equilibrium at all times.
Note that conventional methods of calculating displacement or strains for materials in general from stroke (i.e., the actuator motion) are not accurate. For example, at low strain rate (1/s), strain calculated from stroke tends to overestimate the average strain of the gage section. At high strain rate (500/s), strain calculated from stroke tends to underestimate the average gage section strain. Such conventional methods give inaccurate measurements that need to be filtered and cannot provide data for small strains and for high strain rates. Furthermore, at high strain rates conventional methods results in the sample being difficult to control and an increase in measurement problems as the speed is increased.
Thus, there is a need for new testing methodologies and material information for the strain rates of interest in vehicle design when lightweight materials are employed. In particular, systems and methods need to be developed that can measure a wide range of strain rates from low to intermediate to high rates. It is envisioned that such systems and methods would employ multiple types of sensors, wherein one type of sensor would be configured to measure one range of rates and other types of sensors would be configured to measure other ranges of rates. In such a system, there would be a transition going from one type of sensor to another. Other parameters to be measured by new methods and systems would be: 1) strain-interrupted tests at high rates, 2) methods for characterization of material property degradation (damage) evolution under high rates, 3) methods for failure characterization at high rates, 4) constitutive models for FEM simulations, and 5) investigating formation and growth of voids using microscopy for strains and strain rates of interest.